I crank down pretty tight, with F-style clamps I go to about 80% of the tightest I could possibly do with my bare hands, I’d say. With clamps with a pin or crank handle, which gives you a lever arm, I guess I go until the bar starts to flex. But this seems like a great experiment since TungOil just closes the joint – I’d like to do 3 glue-ups on identically jointed boards and do a fracture test on all 3. Varying clamp pressure or number of clamps for each. Maybe someone else would like to try? To cold for gluing in my shop this week.

The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I’ve seen diagrams that show 45º but that’s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You’ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

As an example, Titebond II clamping pressure, according to Titebond, is 100-150 PSI for softwoods, 125-175 PSI for medium woods, and 175-250 PSI for hardwoods.

While the clamping pressures, in the article listed by AlaskaGuy, may seem really large, the PSI generated can be much less if the area being glued is very large. So if you’re edge gluing boards that are 1” thick and 10” long, you have just 10 sq-in of area and still need a total of something like 1,500 lbs of pressure. Which you can get pretty easy with a handful of lighter duty clamps.

But if you were gluing up something, much larger, like laminating two boards together, something that may have 100 or more square inches, you probably need every clamp you can lay your hands on to begin to get close to what is recommended.

The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I ve seen diagrams that show 45º but that s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

- Rich

I think the 45 degree thing is not just a rough approximation, I think it has something to do with the way materials react. Though I’m sure it does assume a uniform material. And you’re absolutely right on to bring up using cauls. I think most tend to use them as a way to not crush our projects where the clamp contacts it, but they’re important to spread the clamping force out across the joint. But because of the 45 degrees, you actually need the cauls + stock to be as thick as your spacing between clamps to get truly even pressure.

So if you were edge gluing 6” wide boards, you don’t need cauls if the clamps are less than 6” apart. But if you were gluing something just 1” thick, you probably need cauls.

As an example, Titebond II clamping pressure, according to Titebond, is 100-150 PSI for softwoods, 125-175 PSI for medium woods, and 175-250 PSI for hardwoods.

While the clamping pressures, in the article listed by AlaskaGuy, may seem really large, the PSI generated can be much less if the area being glued is very large. So if you re edge gluing boards that are 1” thick and 10” long, you have just 10 sq-in of area and still need a total of something like 1,500 lbs of pressure. Which you can get pretty easy with a handful of lighter duty clamps.

But if you were gluing up something, much larger, like laminating two boards together, something that may have 100 or more square inches, you probably need every clamp you can lay your hands on to begin to get close to what is recommended.

The width of the boards matters too, not just the length. Since the pressure radiates outward from the clamp (I ve seen diagrams that show 45º but that s probably just an approximation), the wider the board, the fewer clamps you need for a given length. For example, picture gluing two 1/4 inch strips. You ll need a clamp every inch or two, or, better yet, cauls to get even pressure along the joint.

And, I agree with AG, too much pressure will starve the joint. I learned to use just enough to bring the boards together firmly.

- Rich

I think the 45 degree thing is not just a rough approximation, I think it has something to do with the way materials react. Though I m sure it does assume a uniform material. And you re absolutely right on to bring up using cauls. I think most tend to use them as a way to not crush our projects where the clamp contacts it, but they re important to spread the clamping force out across the joint. But because of the 45 degrees, you actually need the cauls + stock to be as thick as your spacing between clamps to get truly even pressure.

So if you were edge gluing 6” wide boards, you don t need cauls if the clamps are less than 6” apart. But if you were gluing something just 1” thick, you probably need cauls.

- clin

And what sophisticated expensive equipment do I need to buy so I know how much pressure I’m applying?

In theory: For the OP’s cutting board, 18” long, say 1 1/2” thick, would make it 27 square inches of glue area. For an average pressure of 250 psi, you then need a total of 27*250 = 6,750 lbs clamping force. So about 12 F clamps (or 6 bar clamps).

Which would make it an F clamp every 1 1/2” ....... which seems over the top to me.

In theory: For the OP s cutting board, 18” long, say 1 1/2” thick, would make it 27 square inches of glue area. For an average pressure of 250 psi, you then need a total of 27*250 = 6,750 lbs clamping force. So about 12 F clamps (or 6 bar clamps).

Which would make it an F clamp every 1 1/2” ....... which seems over the top to me.

- unclearthur

You are aptly named. I’m a degreed engineer, and I have never concerned myself with square inches, PSI or any of that when gluing up a panel. Besides, to complicate matters, what if you used psi clamps (Ψ)? That would throw the psi WAAAY off. It’s a slippery slope. Tread carefully.

I know you guys are engineers but, is it just me or are you guys putting way to much thought into this? What ever happened to, “you can never have too many clamps”? I am an internet trained woodworker thanks in most part to lj’s and youtube and a few others. In the 10K woodworking videos I’ve watched and articles I’ve read I have never once heard this discussed or anybody saying, “This is what happens when something is clamped too tightly”. Clamp it good and tight (not over-tight)with as many clamps as seem appropriate then add a couple if there’s room. If there’s no squeeze out I know I haven’t put enough glue in the joint. If there’s a lot of squeeze out I know I put too much but I don’t take it apart like I might if I thought I put too little glue since, as someone already said, “the glue is stronger than the wood”. I might use a little extra glue with softer woods as they seem to absorb more than hardwoods.

Again, I have nowhere near the skills and experience you guys do but is clamping too tight really a thing?

To answer the OP’s question, IMO 4 should be good over 18”, using a long caul on each side and a clamp at the ends of the seam.

Rich, go ahead and have your way with me as I sure do respect your opinion.

I think the obsession with clamping force is related to the obsession with lots of glue…more is always better, right?

Watching videos on YouTube of some people’s glue up, 90% or more of the glue loaded up ends up squeezed right out of the joint. Look at the glue line on an edge jointed board, it’s almost invisible because the glue isn’t a filler(generally) it’s a bonder.Glue is the strongest part of a joint, especially if there is no mechanical strength at play so if you apply X psi with the clamps and squeeze all the glue out of it you are severely weakening the joint. Another thing to note is that if you use a ton of glue it is going to continue to seep out of the joint a while after you have clamped up so you’re going to have to come back in twenty minutes and re-tighten them again.

So it’s a balance, not too much, not too little. Think about some of the techniques people used to use for joining boards before metal clamps were readily available. Wooden cam clamps, timber dogs and even twisted string loops! I’d wager the majority of those joints are still together today…

In regards to your scenario, assuming the mating surfaces are fitting well then I would be more concerned with even clamping pressure rather than amount of pressure. Three clamps “hand tight” or maybe four if I was unsure about the mating surfaces and I would be more than happy!

I think the more relevant question is how strong does the joint need to be? I’ve also seen articles (FWW has one from 2007) basically saying use a s-ton of clamps (and that is impossible to over-clamp a joint with woodworking clamps, they’re not strong enough.) But I also see articles referring to a rub joint, where you don’t clamp at all.

I have no doubt on similarly prepared boards that the rub joint will fail before the one that is “properly” clamped, but does it matter? Do you need to maximum possible joint strength for a cutting board?

Personally, I tend to use fewer clamps, especially on panels. In the OP’s example, I would use two, and those wouldn’t be tight enough to bow the board. If my boards are well jointed, the glue line is still invisible, and the joint is plenty strong.